Organotin compounds or alkyltins are ubiquitous environmental toxins that have been implicated in cellular death. Unlike other xenobiotic compounds, such as organomercurials and organoleads, alkyltins activate apoptotic cascades at low concentrations. Trimethyltin (TMT) chloride is amongst the most toxic organotin compounds, and is known to selectively inflict injury to specific regions of the brain. Stannin (SNN), an 88-residue mitochondrial membrane protein, has been identified as the specific marker for neuronal cell apoptosis induced by TMT intoxication. This high specificity of TMT makes SNN an ideal model system for understanding the mechanism of organotin neurotoxicity at a molecular level. Here, we report the three-dimensional structure and dynamics of SNN in detergent micelles, and its topological orientation in lipid bilayers as determined by solution and solid-state NMR spectroscopy. We found that SNN is a monotopic membrane protein composed of three domains: a single transmembrane helix (residues 10-33) that transverses the lipid bilayer at approximately a 20° angle with respect to the membrane normal; a 28 residue unstructured linker, which includes a conserved CXC metal-binding motif and a putative 14-3-3ζ binding domain; and a distorted cytoplasmic helix (residues 61-79) that is partially absorbed into the plane of the lipid bilayer with a tilt angle of approximately 80° from the membrane normal. The structure and architecture of SNN within the lipid environment provides insight about how this protein transmits toxic insults caused by TMT across the membrane.
Bibliographical noteFunding Information:
The authors would like to thank Dr. Melvin Billingsley for many helpful discussions and Jana Lewis for helping in protein preparations. Acknowledgement is also made to the donors of The Petroleum Research Fund administered by the ACS for partial support of this research. B.B.-K. was supported by the NIH-Chemical Biology Interface Training Grant (GM-08700). NMR instrumentation at the University of Minnesota High-Field NMR Center in the Department of Biochemistry, Molecular Biology, and Biophysics was funded by the NSF (BIR-961477) and the University of Minnesota Medical School.
- Detergent micelles
- Membrane proteins
- Nuclear magnetic resonance
- Organotin compounds